1. Field of the Invention
The present invention relates to a tandem mass spectrometry system having a first mass spectrometer for separating and extracting certain precursor ions from an ionized sample and a second mass spectrometer for analyzing plural product ions created by fragmentation of the precursor ions.
2. Description of Related Art
In recent years, tandem mass spectrometry systems for obtaining structural information about precursor ions by selecting the certain precursor ions from an ionized sample, fragmenting the precursor ions, and analyzing created product ions have been used for structural analysis of samples.
In such a tandem mass spectrometry system, precursor ions separated and extracted by the first mass spectrometer are made to fragment into product ions spontaneously or forcedly by a fragmentation device. The product ions are introduced into the second mass spectrometer. The kinetic energy Up of the product ions introduced into the second mass spectrometer is found from the relational formula:Up=Ui*(mp/mi)where Ui is the kinetic energy possessed by the precursor ions, mi is the mass of the precursor ions, and mp is the mass of the product ions. Since the mass mp of the product ions is smaller than the mass mi of the precursor ions from which the product ions are produced, i.e., mp<mi, the kinetic energy Up of the product ions has an energy range given by 0<Up<Ui. Correspondingly, the kinetic energy of the product ions introduced into the second mass spectrometer has a similar range of energy distribution.
On the other hand, with respect to the second mass spectrometer, the energy range of the measurable kinetic energy of the product ions is intrinsic to the instrument and thus is restricted. Therefore, it is necessary to adjust the energy range of the kinetic energies of the introduced product ions to a measurable range. One conventional method of this adjustment consists of decelerating and accelerating the precursor ions exiting from the first mass spectrometer to narrow the range of the kinetic energies of the product ions produced by fragmentation. Furthermore, the product ions introduced into the second mass spectrometer can be mass analyzed with higher accuracy with reducing the kinetic energy range and spatial spread. Consequently, the precursor ions from which the product ions are produced are preferably reduced in kinetic energy range and spatial spread. The kinetic energy range assumed when the precursor ions are introduced into the fragmentation device is set to about 10 to 100 eV in cases where the kinetic energies are several to tens of keV.
With the above-described conventional technique, when the precursor ions are decelerated, there are difficulties in narrowing the kinetic energy range and spatial spread of the precursor ions. In particular, the precursor ions having kinetic energies of several to tens of keV show a range of kinetic energies of about 50 to 100 eV in the initial state. When the precursor ions are decelerated to reduce the kinetic energies, if the deceleration is achieved, for example, by an electric field in the same direction as the direction of motion, the kinetic energy distribution is maintained and the kinetic energies are about 50 to 100 eV. If the deceleration is achieved by an electric field orthogonal to the direction of motion, the spatial spread of the precursor ions remains unchanged (see, for example, JP 2000-505589, page 1, FIG. 1). If the deceleration is achieved by an electric field applied at an angle to the direction of deceleration, the spatial spread of the precursor ions is enlarged.
In consequence, it is important to realize a tandem mass spectrometry system in which precursor ions are decelerated while suppressing both the kinetic energy range and spatial spread of the precursor ions.